Blender is "... the open source software for 3D modeling, animation, rendering, post-production, interactive creation and playback. Available for all major operating systems under the GNU General Public License."

Blender is a feature-rich and full-value alternative to commercial 3D-modeling suites like 3ds Max and Maya. Due to its open source character, there are various exporters available that allow you to export your models in a format that is suitable for use in Unreal Editor or the UDK.

NOTES: Please read the caveats on p-fat.net before using this script. There are a few things it doesn't handle that you must do to your mesh before the export, like converting all quads to triangles. This script is still very much "alpha", but seems to work fine for meshes, armatures, and animations.

Blender is open source, hence some community members wrote scripts that can help modelers by fulfilling automated tasks, for example modifying an object under certain criteria or creating a certain kind of model to start from.